The basic process of this type is already well-known. See, for instance, U.S. Pat. Nos. 3,780,082 and 3,816,503. These patents also addressed themselves to the problem of product recovery.
Thus, U.S. Pat. No. 3,780,082 discloses that after termination of the reaction, the reaction mixture is distilled at atmospheric pressure. The distillate thus obtained contains the catalyst and the unused reactants. This distillate is recycled, or re-used, for the preparation of additional amounts of 4-oxocapronitrile. The bottoms residue that remains from such distillation contains the desired 4-oxocapronitrile product. This residue is then distilled at a reduced pressure, and the desired oxo-nitrile product was then recovered as distillate in a virtually pure condition (see e.g., Example IV of U.S. Pat. No. 3,780,082).
The later U.S. Pat. No. 3,816,503 discloses that in this process a difficulty arose in that polymer formation occurred, in the reaction vessel, which impeded continuous operation of the process. This patent further taught that by maintaining the oxygen level in the reaction mixture below about 20 parts by weight per million, the deposition of the polymer-like substance into the wall of the reactor and onto the stirrer could be avoided, and the highest then possible yields could be maintained.
Now, however, further study of said process has shown that when temperatures of over 195.degree. C are used in the atmospheric pressure distillation of the catalyst and the unconsumed reactants, the yield of 4-oxocapronitrile considerably decreased. It has been further observed, however, that when the said resulting reaction mixture is distilled in a column at atmospheric pressure, the bottom temperature of the column will in fact be considerably higher than 195.degree. C. On the other hand, if this distillation was carried out at reduced pressure in order to maintain a bottom temperature below 195.degree. C., and thus to prevent the decrease in yield, it was then found that recirculation of the distillate to the fresh reaction mixture then led again to the formation of a deposit of a rubber-like substance on the reactor wall and the stirrer -- even though the content of dissolved oxygen in the fresh reactants was not changed and was kept below the level of 20 parts by weight per million to obtain a reaction mixture having also a content of dissolved oxygen below said level.
It can thus be appreciated that despite the advances made in the prior art teachings, practical difficulties in the continuous operation of this process and the recovery of product have still been present and have needed to be overcome.